1. Field of the Invention
The invention relates in general to navigation equipment for aircraft and particularly for on-board radar equipment for aircraft.
2. Description of the Prior Art
On board vehicles such as aircraft, it is often desirable to illustrate portions of the terrain using radar independently of optical equipment so as to orientate the aircraft relative to the ground. With the use of a map-like presentation, such as are generally obtained with high range resolution surveillance radar using antennas with high azimuth focusing capability frequently do not give sufficient results and on the contrary a perspective representation of the terrain indicating the altitude and if desired of the gradient of their horizon is desirable so as to provide the observer with information concerning the environment of the aircraft and its movement as would be accomplished with direct optical sighting and if necesary to provide the pilot with obstacle warnings.
It is known from German LP No. 2,543,373 to provide a beam antenna which can be focused in the azimuth direction to approximately 0.5.degree. and having elevation focusing of approximately 60.degree. in or on the aircraft with the beam antenna sharply focusing its radiation in the azimuth direction and covering a greater angular range in elevation and the azimuth scanning of the section of terrain to be surveyed is accomplished in chronological succession by means of scanning the antenna in the azimuth direction such that at each azimuth angle illuminated by the vertical pattern of the antenna, the terrain supplies a mixture of echo signals which differ in the transit time respectively proportional to the range of the respective reflecting elements of the terrain as well as having different Doppler frequencies f.sub.d based on the relationship f.sub.d =f.sub.dmax.alpha. .multidot.cos .epsilon.=(2v.sub.H.alpha. /.lambda.).multidot.cos .epsilon., wherein V.sub.H.alpha. represents the aircraft's velocity in the direction of the selected azimuth, .lambda. is the radar wavelength and .epsilon. is the aspect angle which the terrain is scanned relative to the azimuth velocity component in the azimuth direction for the vertical image display at each azimuth. The echo signals respectively received in successive time range windows and from successive range areas are analyzed as to their Doppler frequency and for purposes of display on the picture screen the light spot is vertically deflected proportional to the respective aspect angle .epsilon. in other words, according to the measure of the magnitude .epsilon.=arc cos (f.sub.d /f.sub.dmax.alpha.), so that given the respective incidence of an echo at its specific aspect angle .epsilon. the light spot has reached a deflection proportional to the aspect angle whereby standard time dependent and thus range dependent gain control of the echo signal is provided so the brightness of the image point is independent of the range of the respective reflecting point.
By connecting the individual azimuth scans adjacent to each other on the picture screen a perspective picture of the terrain can be generated such that the representation of the aspect angle is always related to the azimuth motion component of the aircraft relative to the ground and is not related to the aircraft axis.